To gain a better understanding of the vocabulary definition and the structure used by the CIDOC CRM, experimental mappings of Perseus’ and Arachne’s ar- chaeological artifacts have been compiled. The following section reports about the overall methodology, workflow, and the issues and challenges that have been identified during this process. Once a unified representation of data models has been found, the structure of the internal data model can be transformed into the new structure. This transformation process results in a different data model while preserving the semantics. At the same time, however, the new structure needs to conform to a data model that all parties have agreed on. In the scope of the
project described, the XML code needs to be processed in a way that results in a markup that is compatible with the CIDOC CRM.
The need for mapping internal data to common conceptualizations is a chal- lenge that many cultural heritage projects currently need to face. Consequently, multiple research projects started to study the feasibility of abstract mapping soft- ware that can be adapted to the needs of certain databases. Within the EPOCH network, a mapping tool is being developed called the Archive Mapper for Ar- chaeology (AMA).2 The AMA tool is meant to enable mapping other well known standards to the CIDOC CRM. Unfortunately this approach presupposes that the internal data model of a CH database follows a certain standard which certainly will not be the case within most institutions. Another open-source software frame- work, Building Resources for Integrated Cultural Knowledge Services (BRICKS), for building digital libraries includes an “Archaeological Pillar” with the imple- mentation of the “Finds Identifier.”3 This software includes a mapping tool that is based on XSLT. Both mapping tools function well if either the databases can deliver data object of a certain quality or adhere to a certain standard.
As mentioned above, the CIDOC CRM defines a data model that predomi- nantly focuses on events. However, current documentation practice of Perseus and Arachne is not geared to explicitly record information about events. Neverthe- less, whenever data is recorded about archaeological objects, at least implicitly, this entails various events. So for each attribute that was assigned to a specific data object, at least, there is this assignment event. For each date of creation that is attached to a data object, there must have been a creation or production event. Current documentation practice ignores these events but implicitly records information about them that needs to be extracted.
Kondylakis et al. introduced a mapping language for information integra- tion [36]. It claims to cover the most frequent occurrences of heterogeneity and introduces a specific formalism that can be visualized. Figure 5.1 shows the ap- plication of this language in the context of the current project. This mapping language comprises the introduction of intermediate nodes, contraction and ex- traction of compounds, nesting formerly parallel structures, re-using instances for different mappings and performing conditional mapping. The latter addresses cases where the mapping of one field depends on the value of another field.
The first rule of Figure 5.1 is rather straightforward but demonstrates how mapping is performed. Each record of the Perseus art and archaeology table is mapped to the CRM concept E24 Physical Man-Mad Thing, the field name authorityName maps to the property P47 is identified by and the field-value
2http:// www.epoch-net.org/ index.php? option=com content&task=view&id=
74&Itemid=120.
3There are no extensive publications about this particular mapping tool but a brief introduc-
itself finally maps to the class E42 Object Identifier. This is an example of a simple one-to-one mapping operation. The second mapping rule models the period in which the archaeological artifact was crafted. In CRM terms, this involves a production event, and consequently, mapping rules two and three show how a E12 Production Event is introduced to express the creation date of an artifact. Finally, rule four explains how style information can be mapped; this rule uses the CRM class E17 Style Assignment as an intermediate node and then attaches further information. This is an example of changing a parallel structure to a nested one.
This mapping language is supposed to be applied to data of a certain quality. Thus, the mapping language is not even remotely close to being able to deal with the special characteristics of the involved data models; it covers frequent but simple mapping problems with high quality data sources. For the mapping project, these peculiarities will be dealt with in the section on data quality.
These rules stem from a thorough analysis of the Perseus data model. List- ing 5.2 illustrates what a semi-formal documentation of this analysis process might look like. The first step involved finding a set of fields that together need to be mapped to a different set of fields with a certain structure. Then, to help identify the meaning of a particular field, some representative sample values were extracted and documented. In instances where a database field had several hundred values, a “representative” sample was taken. Then, after consulting the CRM definition document and matching the vocabulary definitions, a first mapping proposal was made and then elaborated iteratively. Finally, the overall process was commented on, and problems were documented for reconsideration in the next mapping iter- ation. Parentheses represent a constant value to be inserted and curly brackets represent the value of a specific database field.
Listing 5.2: Semi-formal mapping documentation.
1 A f f e c t e d f i e l d s : 2 " s t y l e " , " f o r m S t y l e D e s c r i p t i o n " 3 4 Sample v a l u e s " s t y l e " : 5 " A r c h a i s t i c " , " E a r l y H e l l e n i s t i c " , " H i g h C l a s s i c a l " , " H i g h C l a s s i c a l ] " 6 7 S c o p e n o t e " s t y l e " : 8 The e p o c h t o w h i c h t h e s t y l e o f t h e d e s c r i b e d a r t i f a c t b e l o n g s . 9 10 Sample v a l u e s " f o r m S t y l e D e s c r i p t i o n " : 11 " < P > A d e s c e n d a n t o f t h e r i d e r s o f t h e P a r t h e n o n f r i e z e . < / P > " , " < P > B e a z l e y n o t e s t h a t t h e s t y l e i s E a s t G r e e k . < / P > " , " < P > E c l e c t i c w o r k , w i t h a l a t e H e l l e n i s t i c m a l e b o d y , a n d a f e m i n i n e h e a d t y p e < / P > " 12 13 S c o p e n o t e " f o r m S t y l e D e s c r i p t i o n " : 14 P r o v i d e s more f u l l −t e x t i n f o r m a t i o n on t h e s t y l e a s s i g n m e n t . 15 16 P r o p o s e d mapping : 17 P14B . w a s c l a s s i f i e d b y 18 E17 . T y p e A s s i g n m e n t ( S t y l e a s s i g n m e n t o f ) { a u t h o r i t y N a m e } 19 P42F . a s s i g n e d 20 E55 . Type ( s t y l e ) 21 P2F . h a s t y p e 22 E55 . Type { s t y l e } 23 P3F . h a s n o t e 24 E62 . S t r i n g ( f o r m S t y l e D e s c r i p t i o n ) 25
26 O t h e r n o t e s : The f i e l d " f o r m S t y l e D e s c r i p t i o n " s o m e t i m e s c o n t a i n s i n v a l i d XML d a t a . Maybe a h e u r i s t i c d a t a c l e a n i n g t o o l c o u l d s o l v e t h i s p r o b l e m . A d d i t i o n a l l y t h e f i e l d s t y l e c o n t a i n s m i s s p e l l e d w o r d s and a d d i t i o n a l c h a r a c t e r s .
Figure 5.2 shows an UML diagram of the Perseus data model and Figure 5.3 an entity relationship diagram of the Arachne data model. The figures demon- strate how different modeling approaches result in very different data models. While Perseus preferred a clean model with just a few tables based on inheritance, Arachne focused on tight and explicit contextualization of archaeological objects forsaking the clarity of the data model. The latter approach is based on the as- sumption that for archaeological objects, the information does not only lie in the metadata alone but in the qualified links to other objects.
Figure 5.2: The Perseus art and archaeology database UML diagram. As previously mentioned, the Perseus data model relies heavily on inheritance. Therefore, as a test case, we decided to start mapping those database fields that are relevant for all objects (this refers to the class AtomicArtifact). Upon making a mapping the problems that arose while mapping the specific fields were enu- merated. These included semantic dependence, semi-structured and unstructured content, and dirty data. In general, Perseus’ and Arachne’s fields are designed for human viewing, i.e. not for machine processing. This results in low granularity of database fields and poses a challenge to extracting more granular data objects.
The processing instructions of XSLT were deemed to be suitable to cover the cases of heterogeneity that appeared within the Perseus data model. Thus, XSL Transformations were used to implement these mapping rules that were elaborated and documented as set out in Listing 5.2. To comply with current developments in
Figure 5.3: The Arachne ERD diagram
the Semantic Web field, the data was transformed to RDF/XML. This will enable most Semantic Web tools to process the data as shown in section 6.2 on page 59. Listing 5.3 shows a cutout of the XSLT style-sheet that was used for mapping. The RDF wrapper was inserted after line 9. Since all objects stored in the database were mapped to E24 Physical Man-Made Thing, this element is inserted in line 10. After that further templates are called (lines 18 and 19).
Listing 5.3: Mapping implementation as XSLT style-sheet.
1 <?xml v e r s i o n=" 1 . 0 " e n c o d i n g=" I S O - 8 8 5 9 - 1 " ?> 2 < x s l : s t y l e s h e e t x m l n s : x s l=" h t t p : / / w w w . w 3 . o r g / 1 9 9 9 / X S L / T r a n s f o r m " 3 x m l n s : d c=" h t t p : / / p u r l . o r g / d c / e l e m e n t s / 1 . 1 / " 4 x m l n s : r d f=" h t t p : / / w w w . w 3 . o r g / 1 9 9 9 / 0 2 / 2 2 - r d f - s y n t a x - n s # " 5 x m l n s : r d f s=" h t t p : / / w w w . w 3 . o r g / 2 0 0 0 / 0 1 / r d f - s c h e m a # " 6 x m l n s : c r m=" h t t p : / / c i d o c . i c s . f o r t h . g r / r d f s / c i d o c _ v 4 . 2 . r d f s # " v e r s i o n=" 2 . 0 " xml:lang=" e n "> 7 < x s l : o u t p u t e n c o d i n g=" U T F - 8 " i n d e n t=" y e s " /> 8 < x s l : t e m p l a t e match=" Q u e r y C o l l e c t i o n "> 9 <rdf:RDF> 10 <c r m : E 2 4 . P h y s i c a l M a n −Made Thing> 11 < x s l : a t t r i b u t e name=" r d f : a b o u t "> 12 < x s l : t e x t> h t t p : // p e r s e u s . t u f t s . edu / a r t i f a c t /</ x s l : t e x t>
13 < x s l : v a l u e −o f s e l e c t =" r e p l a c e ( / / n a m e , ’ [ & q u o t ; ; \ [ \ ] \ + & l t ; & g t ; ] ’ , ’ _ ’ ) " /> 14 </ x s l : a t t r i b u t e> 15 < x s l : v a r i a b l e name=" a r t i f a c t I D " s e l e c t =" / / S c u l p t u r e A r t i f a c t / @ i d " /> 16 < x s l : v a r i a b l e name=" q u e r y L i n k " 17 s e l e c t =" c o n c a t ( ’ h t t p : / / 1 3 4 . 9 5 . 1 1 3 . 2 0 0 : 8 0 8 0 / e x i s t / x q u e r y / a r t i f a c t 2 i m g . x q l ? a r t i f a c t = ’ , $ a r t i f a c t I D ) " /> 18 <x s l : a p p l y −t e m p l a t e s s e l e c t =" / / s t y l e " /> 19 <x s l : a p p l y −t e m p l a t e s s e l e c t =" d o c u m e n t ( $ q u e r y L i n k ) " /> 20 </ c r m : E 2 4 . P h y s i c a l M a n −Made Thing> 21 </ rdf:RDF> 22 </ x s l : t e m p l a t e> 23 < x s l : t e m p l a t e match=" s t y l e ">
24 < x s l : i f t e s t=" s t r i n g - l e n g t h ( ) "> 25 <crm:P41B . w a s c l a s s i f i e d b y> 26 <c r m : E 1 7 . T y p e A s s i g n m e n t> 27 < x s l : a t t r i b u t e name=" r d f : a b o u t "> 28 < x s l : t e x t> h t t p : // p e r s e u s . t u f t s . edu / a s s e s s m e n t /</ x s l : t e x t> 29 < x s l : v a l u e −o f
30 s e l e c t =" r e p l a c e ( / / a u t h o r i t y N a m e , ’ [ & q u o t ; ; \ [ \ ] \ + & l t ; & g t ; ] ’ , ’ _ ’ ) " /> 31 </ x s l : a t t r i b u t e> 32 < d c : t i t l e> 33 < x s l : t e x t>S t y l e a s s i g n m e n t o f </ x s l : t e x t> 34 < x s l : v a l u e −o f s e l e c t =" / / t i t l e " /> 35 </ d c : t i t l e> 36 <crm:P42F . a s s i g n e d> 37 <c r m : E 5 5 . Type> 38 < x s l : a t t r i b u t e name=" r d f : a b o u t "> 39 < x s l : t e x t> h t t p : // p e r s e u s . t u f t s . edu / s t y l e T y p e /</ x s l : t e x t>
40 < x s l : v a l u e −o f s e l e c t =" r e p l a c e ( . , ’ [ & q u o t ; ; \ [ \ ] \ + & l t ; & g t ; ] ’ , ’ _ ’ ) " /> 41 </ x s l : a t t r i b u t e> 42 < d c : t i t l e> 43 < x s l : v a l u e −o f s e l e c t =" . " /> 44 </ d c : t i t l e> 45 <crm:P2F . h a s t y p e> 46 <c r m : E 5 5 . Type> 47 < x s l : a t t r i b u t e name=" r d f : a b o u t "> 48 < x s l : t e x t> h t t p : // p e r s e u s . t u f t s . edu / s t y l e T y p e</ x s l : t e x t> 49 </ x s l : a t t r i b u t e> 50 < d c : t i t l e> 51 < x s l : t e x t> s t y l e</ x s l : t e x t> 52 </ d c : t i t l e> 53 </ c r m : E 5 5 . Type> 54 </ crm:P2F . h a s t y p e> 55 </ c r m : E 5 5 . Type> 56 </ crm:P42F . a s s i g n e d> 57 <x s l : a p p l y −t e m p l a t e s s e l e c t =" / / f o r m S t y l e D e s c r i p t i o n " /> 58 </ c r m : E 1 7 . T y p e A s s i g n m e n t> 59 </ crm:P41B . w a s c l a s s i f i e d b y> 60 <crm:P2F . h a s t y p e> 61 < x s l : a t t r i b u t e name=" r d f : r e s o u r c e "> 62 < x s l : t e x t> h t t p : // p e r s e u s . t u f t s . edu / s t y l e T y p e /</ x s l : t e x t>
63 < x s l : v a l u e −o f s e l e c t =" r e p l a c e ( . , ’ [ & q u o t ; ; \ [ \ ] \ + & l t ; & g t ; ] ’ , ’ _ ’ ) " /> 64 </ x s l : a t t r i b u t e> 65 </ crm:P2F . h a s t y p e> 66 </ x s l : i f > 67 </ x s l : t e m p l a t e> 68 < x s l : t e m p l a t e match=" r e f "> 69 < x s l : i f t e s t=" s t r i n g - l e n g t h ( ) "> 70 <crm:P138B . h a s r e p r e s e n t a t i o n> 71 <c r m : E 3 6 . V i s u a l I t e m> 72 < x s l : a t t r i b u t e name=" r d f : a b o u t "> 73 < x s l : t e x t> h t t p : // r e p o s i t o r y 0 1 . l i b . t u f t s . e d u : 8 0 8 0 / f e d o r a / g e t / t u f t s : p e r s e u s . i m a g e .</ x s l : t e x t> 74 < x s l : v a l u e −o f s e l e c t =" . " /> 75 < x s l : t e x t>/ Thumbnail . png</ x s l : t e x t> 76 </ x s l : a t t r i b u t e> 77 < d c : t i t l e> 78 < x s l : v a l u e −o f s e l e c t =" . " /> 79 </ d c : t i t l e> 80 <d c : t y p e> 81 < x s l : t e x t>i m a g e / j p e g</ x s l : t e x t> 82 </ d c : t y p e> 83 </ c r m : E 3 6 . V i s u a l I t e m> 84 </ crm:P138B . h a s r e p r e s e n t a t i o n> 85 </ x s l : i f > 86 </ x s l : t e m p l a t e> 87 </ x s l : s t y l e s h e e t>
As mentioned above, just like most cultural heritage databases, Perseus and Arachne do not explicitly record information about events in their data models. Therefore, implicit events had to be extracted to make use of all concepts that the CRM has to offer. The template in line 18 evaluates the Perseus field style and connects the E55 Type to an E17 Type Assessment, in lines 23ff. In line 57 an- other template is triggered that evaluates the Perseus field formStyleDescription to further specify the style assignment. This is the implementation for mapping a parallel structure to a nested one that clarifies the relationships among the fields.
According to the Semantic Web, an Uniform Resource Identifier needs to be attached to each Web resource, or it needs to be defined as anonymous (blank node). If — during the mapping process — a node has been created that needs potentially to be referred to, a URI has been assigned, even to events. Otherwise and anonymous node has been created. Lines 11ff demonstrate the construction of unique and unambiguous URIs for a Web resource. In this example the string
http:// perseus.tufts.edu/ artifact/ has been concatenated with the value of the field authorityName. A unique namespace identifying the Perseus art and artifact database has been attached to a unique identifier that points to a specific artifact. As a consequence, this forms a URI that is unique world wide. The replace function in line 13 has been included to guarantee that the URI does not become malformed due to forbidden characters. A HTTP URL has been chosen to facilitate providing a representation of each object as multiple data-formats. However, this has not been implemented yet. Additionally, lines 32ff demonstrate providing a human readable string along with the URI that is meant for machine processing. Many software tools are aware of Dublin Core tags and can figure out the portion of information that was meant for displaying purposes, including the Longwell browser being used within the project on hand to display mapped data objects.
The Perseus project hosts images of art and archaeology objects within a Fedora repository and also maintains an index to resolve the object identification number to one or more images that depict the same object. For mapping purposes this index has been ingested to an eXist database. Line 12 constructs an URL to an Xquery service that is called at line 14. The result document of this query is then evaluated at line 68.
Listing 5.4 shows the result of the mapping process as RDF/XML that can be validated against the RDFS definition document of the CIDOC CRM. Each mapped object has been equipped with a unique identifier and, additionally, for human readability a Dublin Core tag has been attached. The same has been done to events such as the one defined in line 34. Because different attribute assignments could result in different conclusions about the style of an artifact, this approach prevents metadata from becoming contradictory. This helps with merging infor- mation during the process of data integration, contradicting metadata about an object can coexist.
Listing 5.4: The artifact as RDF/XML that conforms to the CIDOC CRM.
1 <?xml v e r s i o n=" 1 . 0 " e n c o d i n g=" u t f - 8 " ?> 2 <rdf:RDF x m l n s : d c=" h t t p : / / p u r l . o r g / d c / e l e m e n t s / 1 . 1 / " 3 x m l n s : r d f=" h t t p : / / w w w . w 3 . o r g / 1 9 9 9 / 0 2 / 2 2 - r d f - s y n t a x - n s # " 4 x m l n s : r d f s=" h t t p : / / w w w . w 3 . o r g / 2 0 0 0 / 0 1 / r d f - s c h e m a # " 5 x m l n s : c r m=" h t t p : / / c i d o c . i c s . f o r t h . g r / r d f s / c i d o c _ v 4 . 2 . r d f s # "> 6 <c r m : E 2 4 . P h y s i c a l M a n −Made Thing r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / a r t i f a c t / N e w _ Y o r k _ 3 0 . 1 1 . 3 "> 7 <crm:P47F . i s i d e n t i f i e d b y> 8 <c r m : E 4 2 . O b j e c t I d e n t i f i e r r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / i d e n t i f i e r s / 2 3 8 9 "> 9 < d c : t i t l e>2 3 8 9</ d c : t i t l e> 10 </ c r m : E 4 2 . O b j e c t I d e n t i f i e r> 11 </ crm:P47F . i s i d e n t i f i e d b y>
12 <crm:P47F . i s i d e n t i f i e d b y>
13 <c r m : E 4 2 . O b j e c t I d e n t i f i e r r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / i d e n t i f i e r s / N e w _ Y o r k _ 3 0 . 1 1 . 3 3 ">
14 < d c : t i t l e>New York 3 0 . 1 1 . 3 3</ d c : t i t l e> 15 </ c r m : E 4 2 . O b j e c t I d e n t i f i e r> 16 </ crm:P47F . i s i d e n t i f i e d b y> 17 <crm:P48F . h a s p r e f e r r e d i d e n t i f i e r r d f : r e s o u r c e =" h t t p : / / p e r s e u s . t u f t s . e d u / i d e n t i f i e r s / N e w _ Y o r k _ 3 0 . 1 1 . 3 3 " /> 18 <crm:P102F . h a s t i t l e> 19 <c r m : E 3 5 . T i t l e r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / t i t l e / F r a g m e n t a r y _ s t e l e _ o f _ w o m a n "> 20 < d c : t i t l e>F r a g m e n t a r y s t e l e o f woman</ d c : t i t l e>
21 </ c r m : E 3 5 . T i t l e> 22 </ crm:P102F . h a s t i t l e> 23 <crm:P2F . h a s t y p e>
24 <c r m : E 5 5 . Type r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / a r t i f a c t T y p e / S c u l p t u r e "> 25 < d c : t i t l e>S c u l p t u r e</ d c : t i t l e>
26 <crm:P2F . h a s t y p e> 27 <c r m : E 5 5 . Type r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / a r t i f a c t T y p e "> 28 < d c : t i t l e>t y p e o f a r t i f a c t</ d c : t i t l e> 29 </ c r m : E 5 5 . Type> 30 </ crm:P2F . h a s t y p e> 31 </ c r m : E 5 5 . Type> 32 </ crm:P2F . h a s t y p e> 33 <crm:P41B . w a s c l a s s i f i e d b y> 34 <c r m : E 1 7 . T y p e A s s i g n m e n t r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / a s s e s s m e n t / N e w _ Y o r k _ 3 0 . 1 1 . 3 3 ">
35 < d c : t i t l e>S t y l e a s s i g n m e n t o f New York 3 0 . 1 1 . 3 3</ d c : t i t l e> 36 <crm:P42F . a s s i g n e d>
37 <c r m : E 5 5 . Type r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / s t y l e T y p e / H i g h _ C l a s s i c a l "> 38 < d c : t i t l e>High C l a s s i c a l</ d c : t i t l e>
39 <crm:P2F . h a s t y p e>
40 <c r m : E 5 5 . Type r d f : a b o u t=" h t t p : / / p e r s e u s . t u f t s . e d u / s t y l e T y p e "> 41 < d c : t i t l e> s t y l e</ d c : t i t l e>
42 </ c r m : E 5 5 . Type> 43 </ crm:P2F . h a s t y p e> 44 </ c r m : E 5 5 . Type> 45 </ crm:P42F . a s s i g n e d> 46 <crm:P3F . h a s n o t e>The s t e l e i s c r o w n e d by a b r o a d e p i s t y l e 47 s u p p o r t i n g a s h a l l o w , p l a i n p e d i m e n t . F r e l a t t r i b u t e d t h i s 48 f i n e p i e c e , and a n o t h e r i n t h e K e r a m e i k o s ( Athens , 49 K e r a m e i k o s P 1 1 3 0 ) t o t h e work o f t h e same s c u l p t o r , 50 p e r h a p s h i s s o−c a l l e d D e x i l e o s s c u l p t o r . C l a i r m o n t a g r e e s 51 t h a t t h e t h i s p i e c e and t h a t i n t h e K e r a m e i k o s s h o u l d be 52 a t t r i b u t e d t o t h e same hand , and t h i s t h e o r y g a i n s s u p p o r t 53 f r o m H e r b e r t s o b s e r v a t i o n s o f t h e s t y l i s t i c s i m i l a r i t i e s 54 b e t w e e n t h e two monuments and t h e i r
55 i n s c r i p t i o n s .</ crm:P3F . h a s n o t e> 56 </ c r m : E 1 7 . T y p e A s s i g n m e n t> 57 </ crm:P41B . w a s c l a s s i f i e d b y> 58 <crm:P2F . h a s t y p e r d f : r e s o u r c e =" h t t p : / / p e r s e u s . t u f t s . e d u / s t y l e T y p e / H i g h _ C l a s s i c a l " /> 59 <crm:P138B . h a s r e p r e s e n t a t i o n> 60 <c r m : E 3 6 . V i s u a l I t e m r d f : a b o u t=" h t t p : / / r e p o s i t o r y 0 1 . l i b . t u f t s . e d u : 8 0 8 0 / f e d o r a / g e t / t u f t s : p e r s e u s . i m a g e . 7 9 8 8 7 / T h u m b n a i l . p n g "> 61 < d c : t i t l e>7 9 8 8 7</ d c : t i t l e> 62 <d c : t y p e>i m a g e / j p e g</ d c : t y p e> 63 </ c r m : E 3 6 . V i s u a l I t e m> 64 </ crm:P138B . h a s r e p r e s e n t a t i o n> 65 </ c r m : E 2 4 . P h y s i c a l M a n −Made Thing> 66 </ rdf:RDF>
In some cases, the CIDOC CRM insists on assigning identifiers to identifiers. It provides a class called E42 Object Identifier that is meant for attaching additional identifiers such as museum inventory numbers to archeological objects. But each identifier needs its own URI along with the additional identifier string to be attached. This leads to an inflation of identifiers typical for the Semantic Web. However, the generated markup can now be transported to a physical place where it can be used by many Semantic Web toolkits for display or more advanced processing purposes, for example, a triple store system with a faceted browser component such as Longwell.